The present invention relates generally to power modulators and in particular to a power modulator and method of operating a power modulator.
Switched-mode modulators are often used for power supply modulation in radio transmitters as a means to improve the efficiency of RF power amplifiers. In either the envelope tracking (ET) technique, or envelope elimination and restoration method (EER), an efficient switching power supply is used to vary the supply voltage of a power amplifier according to an envelope function. The power amplifier is operated at or near saturation, where its efficiency is optimal.
It is well known that the frequency of the switching power modulator must generally be at least 10 times the modulation bandwidth of the amplified signal so to accurately comply with the envelope function and ease filtering requirements. Where large signal bandwidths are used, the required switching rate may exceed that practically attainable, or may be high enough that significant switching losses are incurred.
In order to address these issues, prior art solutions have attempted to control the supply voltage using a combination of a switch-mode converter stage for gross power level control and a subsequent linear regulator stage for precise power envelope control. Such a prior-art solution is shown in FIG. 1 and described in detail in the International Application WO 00/48306 HIGH-EFFICIENCY AMPLIFIER AND BURST CONTROL. As shown in FIG. 1, amplifier 101 is preceded by linear regulator 103 and switching modulator (or switch-mode converter) 105. Although the above solution does allow for more efficient operation of amplifier 101, a problem exists in that oftentimes clipping occurs in amplifier 101. More particularly, in order to avoid saturation of linear regulator 103 and clipping of the envelope function, VPA must be held less than VSUP. Because the prior-art solution discussed above fails to assure that VSUP greater than VPA clipping often occurs. Therefore a need exists for a power modulator and method of operating a power modulator that prevents saturation of linear regulator 103 and clipping amplifier 101.